1. Field of the Invention
The present invention relates to a radio frequency IC (integrated circuit) device. More particularly, the present invention relates to a radio frequency IC device used in a RFID (radio frequency identification) system.
2. Description of the Related Art.
Previous RFID systems have been developed as a system to manage articles. In this RFID system, a reader/writer generating an induction field and a radio frequency tag (also called a radio frequency IC device) that is attached to an article and that stores predetermined information communicate with each other in a non-contact manner to transmit information. As a radio frequency tag used in this type of RFID system, Japanese Unexamined Patent Application Publication No. 2006-80367 suggests a radio frequency tag including an antenna and a matching circuit placed on a flexible substrate and an IC chip electrically continuous to the antenna and the matching circuit. In this tag, the matching circuit is constituted by forming a loop coil (inductor) on the substrate.
However, in the radio frequency tag described in Japanese Unexamined Patent Application Publication No. 2006-80367, the matching circuit to perform impedance matching between the IC chip and the antenna is exposed on the flexible substrate. This configuration has a disadvantage of causing variations in inductance of the loop coil and in characteristics of the matching circuit depending on the permittivity of the article to which the radio frequency tag is attached or the circumstances of the article. These variations cause transmission loss of signals between the IC chip and the antenna, resulting in disadvantageous degradation of radiation characteristics of the antenna. On the other hand, the usable frequency of the radio frequency tag is determined depending on the length of the antenna. The antenna needs to be designed in accordance with the permittivity and other characteristics of the article to which the antenna is to be attached, which is very inconvenient.